Evaluate the following limits:

a$)\underset{r4}{lim}\frac{r^6}{20}-10r^2$

b$)\underset{q}{lim}\frac{q^{-3}-7q^{-6}}{2q^{-4}+4q^{-3}-10q^{-8}}$

c$)\underset{x1}{lim}\frac{-7x^3+7}{3x^2-x-2}$